1. Field of the Invention
The present invention relates to image forming apparatuses that employs the technique of electrophotography.
2. Description of the Related Art
In electrophotography apparatuses, which are image forming apparatuses that employs the technique of electrophotography, the amount of electric charge on a developing solution changes with changes in temperature and humidity and deterioration of the developing solution with time. If the amount of electric charge on a developing solution changes, its a development value γ changes. A development value γ is a value representing the magnitude of development capability of a developing solution. When the development value γ of a developing solution changes, the capability of the toner (development potential), which is in the developing solution, to get attracted toward an image carrier also changes, thereby changing image density and color reproducibility. To get around this problem, current development value γ of a developing solution is measured and the image forming conditions are changed based on the current development value γ.
Also, in two-component developing devices, unless the toner density in the developing solution is appropriately controlled, an abnormal image with background smudges or image dropping due to carrier attachment occur. To get around this problem, based on information from a toner density sensor measuring a toner magnetic permeability in a developer and information about the printed image area, the toner density is controlled to have a predetermined value.
However, if the toner density is always constant, when the development value γ is significantly changed, control may not be made with the development potential. Here, a reason for uncontrollability with the development potential can be explained by the following problems. That is, for multilevel grayscale in laser exposure, for example, grayscale crush occurs if the development potential is too low. Conversely, if the development potential is too high, the capacity of the power supply has to be increased, thereby increasing cost. If development is performed in an intense electric field, adhesion force is increased to cause a transfer failure.
To address the problems, in a conventional apparatus, as exemplarily disclosed in Japanese Patent Laid-Open Publication No. 2003-91224, when the toner density is normally controlled but the development value γ is significantly off the target, the control target value of the toner density is changed. For example, when the toner density is controlled at 5 weight percent and the development value γ is smaller than the target value by 0.3 mg/cm2·kV, the control target value of the toner density is changed to 7 weight percent. Since the required toner attachment amount can be obtained simultaneously when the development value γ is detected, such as when the toner refilling amount is reduced, the development potential is calculated and used for subsequent image formation.
Meanwhile, when the target value of the toner density is changed, for example, when the target value is changed from 5 weight percent to 7 weight percent as explained above, there are a case where toner is refilled or consumed in advance before image formation (printing) until the toner density becomes the target toner density, and another case where a refill control target value is changed during printing so that the toner density follows the target value with a predetermined time constant. In the former case, the toner density is adjusted as a lead-up to printing, and therefore apparatus downtime (long latency time) occurs. In the latter case, although no downtime occurs, the toner density is changed during printing, thereby changing the image density.